Nuclear facilities use atmospheric monitors which collect particulate matter on filters and measure radioactivity in the environment or in working areas. The precise measurement of the radioactivity in the filters remains a major metrological difficulty raised by the diversity of the factors which are able to degrade the energy of the alpha particles emitted by the collected radionuclides. Preliminary studies show that, even in the successful outcome of the nuclear measurement of filters with a weak surface deposit, in-depth penetration of the aerosols in the filter medium can result in underestimating the radioactivity deposited of a factor 2 (Higby, 1984 ; Geryes, 2007a).
The present study was designed to evaluate the effects of the filtration velocity on aerosol burial depth in fiber filters, and consequently on the increase of the kinetic energy loss of alpha-particles due to the path in additional matter. An experimental approach, based on reducing the thickness of a filter to aid in the escape of alpha particles, makes it possible to quantify the radioactive aerosol deposited in depth of the filter medium. This approach allows us to determine the evolution of the activity penetration in-depth of the filter according to the filtration parameters. The filters are prepared by filtering an aerosol of 3.7 µm or 0.15 µm activity median aerodynamic diameter (AMAD) (with standard deviation of 1.4) containing Plutonium 239 produced in the ICARE facility described by Grivaud et al. (1998) and Ruzer and Harley (2005). The filtration velocity in the range of 10 to 1000 cm.s 1 is controlled by flow rate across the filter. For sampling, different types of fiber filters of 47 mm-diameter flat surface were used. Filters containing radioactive aerosols were measured successively by an alpha spectrometer (PIPS) and then by liquid scintillation technique in order to establish a correction factor to estimate the concentration loss of alpha-emitting aerosols when fiber filters are used. Furthermore, radioactivity in filter layers measured by liquid scintillation allows evaluating the extent to which sampling velocity influences burial depth in filters. The protocols of tests allowing the exhaustive study of the penetration of the aerosols in different media as well as associated nuclear measurement will be presented.